The greatest thing we can form any conception of is the starry firmament made familiar to the mind through the study of astronomy.  No limit to this vastitude has ever been assigned.  Since the beginning of recorded time, the earth, together with the other planets and the sun, has been speeding through interstellar space at the rate of 300,000,000 miles a year, without meeting or passing a single star.  A ray of light, travelling with a velocity so great as to be scarcely measurable within the diameter of the earth’s orbit, takes years to reach even the nearest star, centuries to reach those more distant.  Viewed in relation to this universe of suns, our particular sun and all its satellites—­of which the earth is one—­shrinks to a point (a physical point, so to speak—­not geometrical one).

The mind recoils from these immensities:  let us forsake them, then, for more familiar spaces, and consider the earth in its relation to the sun.  Our planet appears as a moving point, tracing out a line—­a one-space—­its path around the sun.  Now let us remove ourselves in imagination only far enough from the earth for human beings thereon to appear as minute moving things, in the semblance, let us say, of insects infesting an apple.  It is clear that from this point of view these beings have a freedom of movement in their “space” (the surface of the earth), of which the larger unit is not possessed; for while the earth itself can follow only a line, its inhabitants are free to move in the two dimensions of the surface of the earth.

Abandoning our last coign of vantage, let us descend in imagination and mingle familiarly among men.  We now perceive that these creatures which from a distance appeared as though flat upon the earth’s surface, are in reality erect at right angles to its plane, and that they are endowed with the power to move their members in three dimensions.  Indeed, man’s ability to traverse the surface of the earth is wholly dependent upon his power of three-dimensional movement.  Observe that with each transfer of our attention from greater units to smaller, we appear to be dealing with a power of movement in an additional dimension.

Looking now in thought not at the body of man, but within it, we apprehend an ordered universe immensely vast in proportion to that physical ultimate we name the electron, as is the firmament immensely vast in proportion to a single star.  It has been suggested that in the infinitely minute of organic bodies there is a power of movement in a fourth dimension.  If so, such four-dimensional movement may be the proximate cause of the phenomenon of growth—­of those chemical changes and renewals whereby an organism is enabled to expand in three-dimensional space, just as by a three-dimensional power of movement (the act of walking) man is able to traverse his two-dimensional space—­the surface of the earth.